1. Field of the Invention
The present invention relates to a single crystal of a nitride for use as heat sinks, electric and electronic components, such as semiconductors, optical components, components of electric equipment and office automation equipment, and other structural components, and a process for preparing the same.
2. Description of the Prior Art
Processes for preparing a single crystal of a nitride include (a) a process wherein high temperature and pressure are applied to a metal to conduct nitriding (a nitriding process), (b) a process which comprises adding a flux component to a metal or a compound thereof, heat-melting the metal or component thereof, and cooling the melt to precipitate a nitride (a flux process), (c) a process wherein a vapor of a compound of a metallic element is transported and reacted to conduct nitriding (a chemical transportation process), (d) a process wherein a metal or a compound thereof is sublimated to deposit a nitride from a vapor phase (a sublimation process), and (e) a process wherein a metallic compound gas is reacted with a gas of nitrogen or a nitrogen compound to deposit a nitride (a chemical vapor deposition process).
For example, "J. of Crystal Growth", Vol. 34, (1976), pp. 263-279 describes mainly about the synthesis of single crystals of AlN, GaN, or InN by the flux process (b), the chemical transportation process (c), and the sublimation process (d). In this literature, however, there is no description to the effect that a single crystal of a nitride having a size useful as electric and electronic components, such as heat sinks, could have been prepared as a bulk.
Japanese Patent Publication No. 5-12320 (1993) describes a process for preparing an AlN single crystal by the flux process (b). Specifically, it describes that an AlN single crystal having a relatively large size, for example, a size of 8 mm square, could be prepared by adding 20 to 70% by weight of an oxide of an alkaline earth metal as a flux to AlN, heat-melting the mixture in a nitrogen or inert gas atmosphere at 1,750 to 2,100.degree. C., and gradually cooling the melt. Further, it describes that further addition of PbO, Fe.sub.2 O.sub.3, Li.sub.2 O, Na.sub.2 O or the like accelerates the flux effect resulting in the formation of a single crystal having good quality.
Japanese Patent Laid-Open No. 7-277897 (1995) describes a flux process (b) similar to that noted above, wherein an Al alloy (but not AlN) is used as a source and directly nitrided. According to this publication, an Al alloy containing 0.001 to 2% by weight of an alkali metal or an alkaline earth metal can be melted in a nonoxidizing atmosphere at 700 to 1,300.degree. C. to conduct nitriding, thereby preparing an AlN crystal having a size of about 200 to 1,000 .mu.m.
The above Japanese Patent Laid-Open No. 7-277897 (1995) further describes that, in the process described therein, a pressure in the range of from atmospheric pressure to several tens of thousands of atmospheric pressure is useful in the nonoxidizing atmosphere, the presence of a very small amount of oxygen in the atmosphere is indispensable, and the partial pressure of oxygen is preferably about 10.sup.-3 atm (0.76 Torr). The role of the alkali metal or alkaline earth metal is such that the alkali metal or alkaline earth metal traps oxygen as an impurity in aluminum, and the alkali metal or alkaline earth metal per se goes as a volatile oxide out of the system, facilitating the penetration of nitrogen, which is contained in the atmosphere, into the system instead. Therefore, according to this process, the content of the impurity oxygen in the crystal is considerably lower than that in the case of the process described in Japanese Patent Publication No. 5-12320 (1993) noted above.
In the above process, however, as described in the same publication, the amount of the alkali metal or alkaline earth metal should be regulated to be a low value from the viewpoint of avoiding such an unfavorable phenomenon that the crystal is broken by self-generation of heat due to the direct nitriding of aluminum. As described in the working examples, a long period of time of 50 hr is necessary for the synthesis of a crystal having the above size in a nitrogen atmosphere at atmospheric pressure. Further, the preparation of a crystal having a size of about 600 .mu.m requires the use of a high pressure of 50,000 atm. Thus, the process described in Japanese Patent Laid-Open No. 7-277897 (1995) has a large problem of production efficiency.
"Physica Scripta.", Vol. T39, (1991), pp. 242-249 describes a process for synthesizing, according to the high pressure process (a), a single crystal of a nitride of a group 3B-5 element, mainly such as GaN, AlN, or InN, at a temperature up to 1,800.degree. C. and a nitrogen pressure up to 15 kb or by pressurization using a piston cylinder pressurizer to a pressure up to 20 kb. This method, however, has problems associated with satisfactory dissolution of nitrogen in the crystal and control of the temperature, and the size of a crystal prepared as a bulk crystal by the synthesis under high pressure nitrogen is not more than 1 mm square. Further, "Microscope", 22, (1974), p. 279 describes that a silicon nitride single crystal having a size of not larger than 55 .mu.m.times.0.1 mm.times.10 mm is synthesized by the high pressure process (a) at a high temperature of 1,800.degree. C. and a high pressure of 15,000 Torr.
"J. Appl. Phys.", Vol. 61, (1987), pp. 2822 2825. describes a process for synthesizing a single crystal of a nitride by a combination of the high pressure process (a) with the flux process (b), that is, by melting and precipitation under high temperature and high pressure conditions. According to this process, BN as a source material is melted through an LiCaBN.sub.2 solvent under high temperature and high pressure conditions, that is, about 55 kBar (5,400 atm, 4.times.10.sup.7 Torr) and about 1,700.degree. C., and a BN single crystal is precipitated on a seed crystal disposed in a crystal growth section having a temperature below that of the source material section. This precipitation procedure for 30 hr results in the formation of a BN single crystal having a maximum size of 3 mm in width.times.1 mm in thickness. Since the above process requires high temperature and high pressure, it is difficult to increase the volume of a crystal growth chamber, imposing a limitation on an increase in size of the single crystal grown. Further, a large apparatus is necessary for the generation of high temperature and high pressure, unfavorably increasing the production cost.
Kurai et al. of Tokushima University, Proceedings of Electronic Material Symposium in International Conference held on May, 1995, pp. 45-47 introduces the synthesis of a GaN single crystal on sapphire by the sublimation process (d). According to this literature, a sapphire substrate and a GaN powder as a crystal source are placed within a quartz tube and heated in an ammonia atmosphere at 1,100.degree. C. to grow a GaN single crystal having a thickness of 30 .mu.m on the substrate.
Japanese Patent Publication No. 3-53277 (1991) also discloses, as an example of the sublimation process (d), a process for epitaxially growing a single crystal having a composition of (SiC).sub.x (AlN).sub.1-x, wherein x is 0.2 to 0.5, on a substrate, which comprises placing Al.sub.2 O.sub.3 and SiC as crystal sources within a carbon or tungsten crucible, disposing a substrate, such as sapphire, W, or SiC, with a given interval, and heating the crystal source in a mixed gas stream composed of 85% of nitrogen gas and 15% of hydrogen gas to 1,900 to 2,020.degree. C. In this case, hydrogen gas serves to accelerate the growth of the crystal, the gas is flown from the crystal source towards the substrate, and the substrate is kept at a temperature 10 to 100.degree. C. below the temperature of the crystal source. The size of the crystal prepared by this process is described to be about 1 cm.sup.2 in area and about 1 to 10 .mu.m in thickness.
Japanese Patent Publication No. 62-51240 (1987) describes a process for preparing an SiC single crystal by the sublimation process (d). According to this process, a mixture of 90 to 40% of SiC and 10 to 60% by weight of a silicon material, for example, silica sand, is placed in a graphite crucible, and the mixture is heated to 2,300.degree. C. or below in an inert gas atmosphere at a pressure of 100 to 1,500 mm in terms of water-gauge pressure (754 to 870 Torr) to reprecipitate an SiC single crystal in the crucible. According to the description of this publication, although a plate crystal having a size of 2 to 9 mm square could be prepared, it is difficult to grow a crystal at a gas pressure outside the above range.
"Yogyo-kyokai-shi (Journal of The Ceramic Society of Japan)", 81, (1973), pp. 441-444 describes, as an example of the sublimation process (d), a process wherein the wall of a graphite vessel is coated with an Si.sub.3 N.sub.4 powder, which has been prepared by nitriding high-purity Si, to form an Si.sub.3 N.sub.4 layer which is then sublimated at 1,700.degree. C. for 7 hr to prepare an Si.sub.3 N.sub.4 single crystal on the internal surface of the vessel. The Si.sub.3 N.sub.4 single crystal prepared by this process is a pale brown, hexagonal columnar single crystal having a length of about 100 .mu.m and an outer diameter of about 20 .mu.m, that is, very small.
Regarding the publications other than described above, "J. of Crystal Growth", 21, (1974), pp. 317-318 describes that high-purity silicon is melted under a nitrogen gas stream of one atm at 1,650.degree. C. for 6 to 12 hr and a .beta.-Si.sub.3 N.sub.4 single crystal is synthesized from the melt. The resultant Si.sub.3 N.sub.4 single crystal is acicular and has a diameter of 0.1 to 0.3 mm and a length of several mm.
"Powder Metallurgy International", Vol. 16, No. 5, pp. 223-226 describes a chemical vapor deposition process (e) wherein a source material gas composed of SiCl.sub.4, NH.sub.3, and H.sub.2 is fed at 50 torr to synthesize a silicon nitride single crystal having a width of 2 mm, a length of 3 mm, and a thickness of 0.5 mm on a graphite heated at 1,500.degree. C. "J. Mater. Sci.," 14, (1979), p. 1952 describes that a single crystal of .beta.-Si.sub.3 N.sub.4 having an approximate size of 2.times.3.times.0.5 mm is synthesized by a CVD process using a mixed gas of NH.sub.3 +SiCl.sub.4 +H.sub.2 under a temperature of 1,400 to 1,600.degree. C. and a pressure of 1.5 to 7.5 Torr.
The synthesis of an Si.sub.3 N.sub.4 single crystal by the chemical vapor deposition process (e) is reported in publications other than described above. For example, "J. of Crystal Growth", 24/25, (1974), pp. 183-187 describes that, by CVD using a mixed gas of H.sub.2 +SiCl.sub.4 +N.sub.2, a polycrystalline Si.sub.3 N.sub.4 is deposited at 1,250.degree. C. on a graphite and heated to be grown into a single crystal. According to this process, an Si.sub.3 N.sub.4 single crystal is synthesized by CVD at 1 atm. The resultant single crystal, however, is small and has an approximate size of several mm in length.times.0.1 to 0.3 mm in diameter. Further, "J. Am. Ceram. Soc.", 79 (8), (1996), pp. 2065-2073 describes that an .alpha.-Si.sub.3 N.sub.4 single crystal, which is several mm in length and diameter, is synthesized by CVD using a mixed gas of NH.sub.3 +HSiCl.sub.3 +H.sub.2 under a temperature of 1,300 to 1,500.degree. C. and a pressure of 0.5 Torr.
As described above, processes for preparing a single crystal of a nitride include (a) a high pressure process, (b) a flux process, (c) a chemical transportation process, (d) a sublimation process, and (e) a chemical vapor deposition process. The high pressure process (a), however, is disadvantageous in that it is difficult to regulate the content of nitrogen in the crystal, a bulk crystal having a size large enough to be suitable for practical use cannot be prepared and, in addition, the production cost is high. In the case of the flux process (b), a relatively large crystal having an approximate size of 8 mm square can be prepared. In this process, however, a lowering in purity of the crystal caused by a flux, for example, in particular, inclusion of oxygen in the crystal, is unavoidable, and any crystal having good quality capable of meeting property requirements for practical use cannot be prepared. Further, the process, similar to the flux process, described in Japanese Patent Laid-Open No. 7-277897 (1995) can minimize the inclusion of oxygen. As described above, however, the problem of the production efficiency remains unsolved. In the case of the chemical transportation process (c), the source material cost and the production control cost are increased, and, at the same time, the orientation of the resultant crystal is poor. Therefore, crystals prepared by this process have poor suitability for practical use, and crystals usable for practical use are limited to very thin crystals.
The sublimation process (d) can offer a relatively large crystal having good quality. In this process, however, the control of conditions in the crystal growth section is difficult, and, in addition, the sublimation efficiency is low, making it difficult to prepare a large single crystal. Thus, a single crystal of a nitride having a size large enough to be suitable for practical use as a heat sink, an optical component, a semiconductor, a structural material or the like cannot be prepared as a bulk material. The chemical vapor deposition process (e) has a problem of safety because a source material gas having high activity should be handled at a high temperature for a long period of time.
In particular, in the case of the sublimation process (e), use of a large seed crystal as a substrate for growth of a crystal is preferred from the viewpoint of growing a large single crystal. At the present time, however, any large crystal of a nitride usable as a seed crystal cannot be prepared, and other single crystals are used as an alternative. However, there are only a few single crystals utilizable at a high temperature under which a nitride is grown. Even though the single crystal is utilizable, it is, in many cases, difficult to prepare a large single crystal necessary for growing a large single crystal of a nitride. Further, in the conventional sublimation process, since the growth of the crystal is carried out in a high-temperature an atmosphere containing nitrogen, a reaction is likely to occur in a single crystal other than that of a nitride, rendering the crystal unusable as the seed crystal. For example, an attempt to grow aluminum nitride using silicon carbide as a seed crystal results in decomposition of silicon carbide causing sublimation. Thus, in the conventional sublimation process, the temperature is so high that it is difficult to utilize a seed crystal.